1. Field of the Invention
The subject invention pertains to the field of thermoplastic syntactic foams. More particularly, it pertains to a process for preparing thermoplastic syntactic foams by consolidating stable compositions containing fine thermoplastic powder and microballoons dispersed in a non-solvent.
2. Description of the Related Art
Syntactic foams differ from blown foams such as polyurethane foam and expanded polystyrene foam in that the cells in syntactic foams are preformed by way of incorporation of small diameter hollow spheres rather than by the expansion of a blowing agent.
It is known to prepare syntactic foams from thermosetting resins such as unsaturated polyesters, bismaleimides, non-cellular (no blowing agent) polyurethanes, cyanates, thermosetting polyimides, and the like. When the uncured resin is of sufficiently low viscosity, the appropriate microballoons are merely mixed into the resin. When the resin system is too viscous or a solid, then a solvent is generally added. The solvent is selected to be one in which the resin system components are soluble.
Thermoplastic syntactic foams are also known. Such foams have been prepared by admixing in dry form, microballoons and thermoplastic powder followed by consolidation as disclosed in U.S. Pat. No. 4,876,055; or by adding microballoons to a melt of thermoplastic in a kneader or extruder. When thermoplastic powder and microballoons are utilized, these compositions have the disadvantage of difficulty of handling due to the dust created when transferring or pouring, and the ease with which the compositions may be disturbed by air currents, for example those created by forced air ventilations. Such compositions may also be prone to settling, with the higher density thermoplastic particles settling to the bottom of the container, creating variations in composition.
It is further known to utilize a thermoplastic dissolved in a solvent. An example of this method is illustrated by U.S. Pat. No. 4,077,922, wherein an aprotic solvent is utilized to dissolve the thermoplastic, in this case a polyimide. Also known is the use of a combination of an aprotic solvent in conjunction with a dissolution-preventing non-solvent. In this case, the non-solvent is removed prior to subjecting the mold to elevated temperature. Following its removal, the aprotic solvent then solubilizes the polyimide. Both these methods require the use of expensive aprotic solvents which may further create toxicity problems during removal. The second method described above requires a two stage process not well adapted to commercial production. Finally, since the aprotic solvent is soluble in the thermoplastic matrix, its complete removal is problematic.
When a melt of thermoplastic is utilized, the density of the microballoons must generally be high to avoid breakage during the mixing process. When extrusion is utilized, for example, microballoons having densities higher than 1.00 g/cm.sup.3 and wall thicknesses of at least approximately 10 percent of their diameter must be used. See, for example, Plastics Compounding, Mar./Apr., 1981 which describes rapid loss of microballoons by crushing under high shear mixing conditions.